1. Technical Field
The present invention relates to an optical system; and more particularly, to an optical system for providing athermal sensing of changes in a Bragg grating.
2. Description of Related Art
The majority of prior art athermal Bragg grating designs rely on tensioning in-fiber Bragg gratings via co-linear or concentric coatings or inserts. Performance of these designs is highly dependent on the repeatability of the manufacturing process, as well as on the thermomechanical properties of the coatings, adhesives, inserts, etc. In addition, these designs do not afford a post-manufacturing adjustment to accurately tune the response of the grating to precisely and repeatably achieve the desired temperature sensitivity specifications. As a result, more stringent manufacturing is required, thus leading to reduced yields and limited performance. Finally, none of the current technologies take advantage of simple levering concepts to widen the athermal grating design space.
The present invention provides an athermal grating design having a Bragg grating unit and a lever arrangement.
In operation, the Bragg grating unit responds to an optical signal, a change of temperature and a lever force for offsetting thermally-induced changes in the Bragg grating unit, for providing a Bragg grating unit signal that does not change in relation to the change of temperature.
Further, the lever arrangement responds to the change of temperature, for providing the level force to the Bragg grating unit to compensate for the change in the temperature.
The Bragg grating unit may include a large diameter waveguide cane structure.
The lever arrangement may include a top plate, a bottom plate, a lever arm pivotally coupled between the top plate and the bottom plate, and a rod coupled between the top plate and the bottom plate on one side of the lever arm. The Bragg grating unit is arranged between the top plate and the bottom plate on another side of the lever arm.
The level arm and the rod have different coefficients of expansion. The level arm is adjustably coupled between the top plate and the bottom plate. The level arm is attached to the top plate by a pin joint, and is attached to the bottom plate via a threaded nut and screw arrangement.
The threaded nut and screw arrangement may include a nut or other fastening device that is tightened to compress both the rod and the Bragg grating unit.
The rod is adjustably coupled between the top plate and the bottom plate. As shown, the rod includes a threaded post having course threads for coarsely adjusting the pre-compression of the rod between the top plate and the bottom plate. The rod includes a threaded rod having fine threads for finely adjusting the pre-compression of the rod between the top plate and the bottom plate.
The lever force for offsetting thermally-induced changes in the Bragg grating unit depends on and is determined by the coefficient of thermal expansion of the lever, rod and Bragg grating unit; the Young""s modulus of the lever, rod and Bragg grating unit; cross-section of the lever, rod and Bragg grating unit; the lengths of the lever, rod and Bragg grating unit; the distance between the lever and the rod; or the distance between the lever and the Bragg grating unit, or a combination thereof.
In one embodiment, the coefficient of thermal expansion of the lever is less than the coefficient of thermal expansion of the rod so as to tune the Bragg grating unit by compression. In another embodiment, the coefficient of thermal expansion of the lever is greater than the coefficient of thermal expansion of the rod so as to tune the Bragg grating unit by tension. In this case, the Bragg grating unit may be an optical fiber having a bragg grating therein.
The foregoing and other objects, features and advantages of the present invention will become more apparent in light of the following detailed description of exemplary embodiments thereof, as illustrated in the accompanying drawing.